1. Field of the Invention
The present invention generally relates to a method of controlling engine operation and more specifically relates to such a method adapted for use with a marine engine designed for stoichiometric and lean burn operation.
2. Description of the Related Art
Internal combustion engines are used to power various types of vehicles. For instance, land vehicles, such as automobiles, are powered by internal combustion engines. Such engines can be designed to operate under various air-fuel ratios.
With reference to FIG. 1, operation of one such engine design used in land vehicle application is graphically depicted. In the illustrated arrangement, intake air pressure is shown as a function of throttle opening. The air-fuel ratio, in turn, is shown as a function of throttle opening as well.
As known, the engine operates at an idle speed when the throttle valve is totally or substantially closed. At idle speed, the intake air pressure at a location between the throttle valve and the combustion chamber is at a minimum. Thus, during idle, the engine is operating in a low speed and low load mode. This low speed and low load mode generally continues as the vehicle slowly accelerates and cruises at highway speed. Of course, short bursts of higher speed and higher load operation can be expected.
The low speed and low load operating range, nevertheless, is the normal operating range for an automobile. From time to time, the engine may be called upon to operate within a high load and high speed operating range, albeit fairly infrequently. With reference to FIG. 1, the illustrated arrangement provides that the engine transitions out of a lean burn mode after the intake air pressure has reached a maximum air pressure, which is associated with high load operation.
As illustrated, when operating the engine in other than the high load and high speed operating range, it is possible to operate the engine in a lean burn mode. The lean burn mode involves supplying a lower than stoichiometric air-fuel ratio, which supplies less fuel per combustion cycle. Such lean burn operation, therefore, can lower fuel consumption. When engine demand is great, however, the air-fuel ratio can be richened to provide for better response to operator demand.
Marine vehicles, on the other hand, generally operate in the high load and high speed operating range once the transmission engages a propulsion unit (e.g., propeller or jet pump). Thus, the engine spends a majority of its run time in a high load operating range. As such, the above-described lean burn transition would result in minimal fuel conservation.
One thought is to design the engine to operate in the lean burn mode. Such an engine design adds significantly to the cost and complexity of the engine design. For instance, devices such as a swirl control valve or a valve stop and design changes such as a helical port or a tumble port would have to be integrated into the construction of the engine for proper operation in lean burn mode at all times.
Thus, a marine engine that is capable of reducing fuel consumption by operating in a lean burn mode yet capable of improved stability during idling and trawling operation is desired.
Accordingly, one aspect of the present invention involves an outboard motor for a watercraft. The outboard motor comprises an engine body defining at least one cylinder bore in which a piston reciprocates. A cylinder head is affixed to one end of the engine body and closes the cylinder bore and defines with the piston and the cylinder bore a combustion chamber. An intake passage is in fluid communication with the combustion chamber and is configured to provide air for an air/fuel mixture to the combustion chamber. A throttle body is in fluid communication with the intake passage and has a throttle plate configured to control an airflow in the intake passageway. A throttle position sensor is configured to determine a position of the throttle plate. An intake air pressure sensor is in fluid communication with the intake passage. The air pressure sensor is positioned between the throttle valve and the combustion chamber and is configured to determine air pressure in the intake passage. A fuel injector is configured to deliver fuel to the combustion chamber for the air/fuel mixture. An engine speed detector is configured to determine an engine speed. An engine control unit is configured to control the fuel injector based upon feedback from at least one of the throttle position sensor, the engine speed detector, and the intake air pressure sensor. Between a closed state of the throttle plate and a first predetermined air pressure, a constant air/fuel ratio is maintained. From the first predetermined air pressure to a second predetermined air pressure, the air/fuel ratio is steadily increased as a function of a change in air pressure to approximately a lean limit ratio. The second predetermined air pressure is less than a maximum intake air pressure. The maximum intake air pressure occurs when the air pressure in the intake passage becomes approximately constant. From the second predetermined intake air pressure to the maximum intake air pressure, the air/fuel ratio is maintained at approximately the lean limit and from the maximum intake air pressure to a maximum throttle opening, the air/fuel ratio is decreased in accordance with feedback from the throttle position sensor and the engine speed detector.
Another aspect of the present invention involves a method of operating an outboard motor. The outboard motor comprises an engine driving a marine propulsion device at speeds indicated by an engine speed sensor. The method comprises detecting an induction system air pressure at a location between a throttle valve and a combustion chamber, supplying a preset constant air/fuel ratio to the combustion chamber at sensed air pressures lower than a first predetermined air pressure, supplying a variable air/fuel ratio at sensed air pressures between the first predetermined air pressure and a second predetermined air pressure, supplying a lean limit air/fuel ratio at sensed air pressures between the second predetermined air pressure and a maximum air pressure and supplying a variable air/fuel ratio at throttle angles greater than a minimum throttle angle corresponding to the maximum air pressure.